The proposed research is directed toward the differences that exist in the regulatory as well as structural properties of phosphofructokinase in tissues of diverse metabolic function. Particular focus will be placed on the regulator properties of a new isozyme found in brain and on the comparison of its properties with other isozymes. Further studies of isozyme hybridization under conditions that might exist in vivo will be carried out. The regulatory properties of the principal hybrid, A2B2, will be examined.